Anyone who's watched a lot of Improv will be able to tell you that there is some sort of indefinable quality that makes some improvisers more watchable on stage than others. This has nothing to do with talent or experience, but it's just a quality that's there that makes it difficult to take your eyes of someone.
I'm pretty sure Keith Johnstone (I can't find the exact reference in his books) said that an improvisor should try and make the audience want to take them home at the end of the evening. Whether this is for the same reason you'd adopt a kitten or for more morally corrupt reasons I can't recall, but I think this is a good aim.
We make snap judgements in everyday life, I know for one that when I meet someone I decide in the first 10 seconds or not whether I like them of not. Likewise with art, we often just know if something is good or if we like it, regardless of what sort of art background we have. Another example, would be looking at flats, from experience the worst flats I have lived in have been ones where my gut told me this wasn't right yet my analytical mind over-ruled this initial instinct. More often than not these instant judgements are correct, but why? I don't think anyone really knows, but millions of years of evolution probably has something to do with it.
But this doesn't answer the questions as to what makes some improvisers more watchable on stage than others? I've though about this a lot recently, and although I can't put my finger on it, and here are a few traits which I think more watchable improvisers have, you'll note that I'm more certain about some things rather than others:
- Does not appear to dominate scenes, gives the other actors space;
- Not afraid to make mistakes, constantly pushing themselves and getting themselves in trouble;
- Plays slightly low status to the audience;
- An egoless performer (conversely an ego driven performer is nigh on unwatchable), unaware of how good they are;
- Happy to play furniture, make a small addition to the scene;
- Never look like they are trying too hard, make things look effortless, as if they are not even thinking;
- Emotionally honest with themselves, they allow a little peak into their or their character's soul;
- Can see their enjoyment, they look like they want to be on stage.
By no means is this a full and thorough list, but I do think it answers a very small segment of the question (or at least serves as a useful list of performance skills). In fact I am probably more confused now as to what works than when I first started writing this what I like and what I don't. I also know that this doesn't even gets close to identifying what it the watchability factor actually is.
It should probably be pointed out that I don't want to give the impression that other improvisers are unwatchable. This certainly isn't the case, what I'm getting at, and I'm going to paraphrase Orwell - "All improvisors are watchable, but some are more watchable than others".
Perhaps we shouldn't think too hard as to why we like something and just accept it for what it is as some things just can't be answered.