I have written about looping animation and walk cycles before, accentuating on the importance that the well-designed internal symbol structure has for achieving optimal flexibility and control.
I have also written at length about how with digital cutouts we can produce rather organic-looking animation, which does not suffer from the typical floatiness and disjointedness of Flash-animated puppets.
In this article I'd like to share a few more thoughts and practical advice on the topic.

The importance of a well-developed walk cycle should be obvious to anyone interested in animation. A walk is a potent tool to introduce a character, fill in time, convey mood, advance the story, provide rhythmic visual pleasure and so on. Because of its central role and its relative complexity, a lot of instruction material from various authors has been dedicated to the walk cycle.

Even the simplest and most basic walk-on-spot presents beginner animators with plenty of challenges. Working with a timeline instead of drawing on separate sheets of paper also adds some specific problems, which are poorly understood, and seem to never be discussed. There is a very difficult to define difference between how we think about four physical drawings which we can spread on the desk in front of us and look at simultaneously, and the four keyframes on our screen, which always appear in a certain order on the timeline.
While instant preview is surely empowering, it is also restraining to our thought process as it merges posing and timing into one, while in essence they are two separate interconnected processes.

As we advance further into the digital age, it is becoming more and more difficult to try and persuade young aspiring animators to build lightdesks and spend some time animating on paper. The results won't be so instantly gratifying, but will help them develop a sense of what a pose and a frame actually mean, and how they relate to time.
Animating several walk cycles frame-by-frame, making all the mistakes that can be made and having first-hand experience with all the difficulties in organizing even the most basic walk will let the animator truly enjoy the liberation that animating digital cutouts can bring. Possessing good knowledge of the basics and clear methodology how to approach the task let us be playful, experiment and enjoy animating walks.

The two videos below demonstrate an error-proof method of structuring containers for flexibility and experimentation. I try to present the process in a manner that is both simple and logical.
As I have said multiple times, how we do things greatly affects what we can achieve – certain structures facilitate better animation, while others literally prevent it.

Once the basics are internalized, only then the fun can begin.

Basic rules which are often neglected

One of the most basic rules for a normal, healthy walk cycle is that it should always consist of an even number of frames – each of the two steps in the cycle happens over the same amount of time. Breaking this will obviously lead to a limp.

The trajectories of left and right feet have to be symmetrical and the two steps have to be of the same perceived length.
The same applies to body and hand movements.

Another insurmountable rule is that from the moment a foot touches the ground to the moment is loses contact it must travel evenly, i.e. no easing in or out should ever be applied to the contact foot. The distances between consecutive frames should always be the same.

The personality and mood of the walk comes from everything else, not from the planted foot. It is on the ground, it supports the body weight and that's it.

Flexibility and fun

One of the best things about animating digital cutouts is that we can experiment quickly and easily.
When the setup is optimal we can only animate one leg, but see the whole walk, experiment with varied timing and quickly change or tweak the key poses. We can also organize our preview to show how the cycle would look like when tweened across the screen or include a soundtrack and time it to music.

This can be a viable approach even to preparing rough guides for frame-by-frame animation. 

Secondary actions

Once the big problems are solved, we can work on secondary and tertiary movements. Many such movements (gentle head turns, hair wobble) can be added inside the synced graphic containers.

If the cycle will stay on screen longer and be in a relatively static position with a panning background behind, it is worthwhile developing the secondary actions more.
If it will be tweened across the stage, many subtle movements will be lost to the viewer, so it is probably unnecessary to add them at all.

Real world examples with commentary

With all the above information in mind, let's now look at some walks and cycles I animated for a children's series, called The Animal Alphabet. Many of them were animated to specific music and beats.

I animate cycles separately and then integrate them into the actual shot, but in most cases I pose the character in the shot first, then isolate this initial pose and animate in a new document.
While it is always nice to have a beautiful walk cycle that works well in isolation, the actual goal is for it to integrate flawlessly with the other animation. This means that often there is some back and forth, until the timing and stride are just right.

Out of the whole series within the given production period I could single-handedly animate 9 episodes plus the title sequence. The rest were outsourced to a cheap overseas service studio and the quality varies from decent to shamefully bad, even within the same episode. Apart from writing an earlier version of Organic Animation and Cinematic Look and providing all character designs and Smart Magnet Rigs, unfortunately, I had absolutely no say over what is acceptable and what is not quality-wise.

Clicking on the links below the GIFs will pop the video of the final shots, where the cycle was used.

'O' was the last episode, which I animated. By that time all the techniques were perfected and the style of the show was very well-established. To keep myself interested I still pushed the boundaries a bit, but that was mostly in the continuity between the segments and the acting of the characters, experimenting even more with timing everything to the music and its beat. This particular walk cycle is well-articulated and competent, without being experimental in any way – just a good introduction of the ostrich. Finished Scene

The otter had a couple of different cycles. This one was designed to not stay static in the frame for too long, that is why I did not spend too much time perfecting all the secondary details. When combined with special effects and camera movement its main dynamic is good enough to make it look convincing. Finished Scene

The chameleon was one of the challenging characters to set up due to my desire to be able to do all kinds of aesthetically pleasing color changes timed independently of the character acting. This walk is somehow basic, but it has certain mood and character. It served well as a foundation upon which to develop the animation. Finished Scene

The fun with the kitten was that it mimicked several different animals which it met in the garden (watch the short video). This particular hop cycle heavily favors the apex position, slowing in and out of it, making both characters lightweight and springy. Finished Scene

The jacana has a very unique anatomy, which dictated the logic of the normal walk. 

Having it walk and juggle jelly beans at the same time was really an interesting and unusual task – how to balance the whole figure, so that it all looks convincing. Just like the chameleon above, organizing the symbols in a meaningful way allowed me to combine all these complex and varied in speed movements in a whole which does not melt or disassemble. Finished Scene

The alligator turned out to be a surprisingly flexible and enjoyable rig to animate. Its proportions and body structure determined the kind of actions it could perform. Finished Scene

These seagulls were just a silly choir, added to bring joy and audio variation to a little dance animation. I have seen seagulls do these head bobbing displays to each other, so I thought it could be fun to try and emulate the behavior in animation. Finished Scene

A springy hop on spot by the jerboa – a small desert rodent. Small animals are usually quite restless, so I wanted to give it a bit of a hyper-active behavior and never keep it static for too long. That is why I went for such a hop cycle. Finished Scene

A double-bounce walk for the diplodocus, which I could also reuse as a little dance by just reversing the animation in a ping-pong sequence – something which very rarely works. Finished Scene

Another double-bounce walk – this time for the heron. It works well with the jazzy tune, but I somehow couldn't use it to its full potential in the actual shot. It's a bit poorly staged, and the funny 'gag' which I envisioned with the 'flat' front view of its face didn't work well at all. I must have been under pressure to get it done as quickly as possible, so I didn't have the time and energy to rework the whole, but the walk itself is actually not bad. Finished Scene

'C' was one of the earliest episodes and the camel is probably the first double-bounce walk I did for this show. In the later episodes the character designs got a little more complex and the visual style a little more polished. Finished Scene

One more double-bounce walk. In the actual shot I even made the duckling open and close its eyes on the beat. I don't know if this kind of stuff is noticed by anyone, but is very enjoyable to do. Pay attention to it while watching the video. There is even a double-bounce swim cycle at the end. Finished Scene

The jellyfish consists mostly of animated shape tweens. It was another one of these unusual and interesting tasks, which make working on an animal-themed show so varied. Of course, it was quite challenging how to fill in all the necessary time with just one looping animation of a beautiful, but relatively passive life form. Finished Scene

The meerkat marching band was a prominent feature of the intro sequence. Again, animated to the beat of the music. Finished Scene

The snail is one of those animals that don't have much joint articulation for cutouts and you just have to make things up as you go. Finished Scene

A springy bounce of an antelope with a lot of time spent in the air and very little on the ground. They do hop a bit like this, and it is both typical and pleasant to observe. Finished Scene

This is a stretched and slowed down double-bounce with a 'pause' in the middle – just something I made up as I wanted to give the horse some personality and introduce him in a memorable way. Finished Scene

Just a fun hop cycle of the toucan, which was used a few times in various shots. Finished Scene

A cute 24-frame dance cycle, timed in the same way as a double-bounce walk. Any bulky character with small legs and arms will probably look as cute as this. It comes from the proportions, combined with some perceived agility and finesse. Finished Scene