Driving home this afternoon, I was pondering life, the universe and everything. Didn't come up with "42", but did realize a pseudo-logical connection between a performer's insecurity and the hunger for bigger, better roles. It's all about being noticed - when you're the 3rd lovesick maiden from the left, 2nd row back, and all of you are dressed in aesthetic pastel gowns, about the only people who'll notice you are the audience members who know you. If you're the character whose only function is to answer the phone in 2 or 3 scenes and tell the ingenue "It's him again", you're likely to feel more like a piece of the set than a member of the cast.
On the other hand, if you've got lines, or a big scene, a solo or a bit of special "business", the audience is likely to notice. Mae Peterson is remarkably nasty to Rosie; it was fun to hear the audience gasp at some of the vicious digs she made. ("What does Rosie need a job for? In a year or two she'll be on Social Security.") When I played Pitti-Sing, I could be more of an individual than when I've been a chorister. No worries about pulling focus; as long as I shared it with the other leads on stage, I was "legal".
I've almost never gotten comments on my performance when I've been a chorister or had a minor role (although "Sr. Mary Velcro" was a lot of fun!), and then it's been along the lines of "you moved well in the choreographies" or "you were always so engaged in the action". Which is certainly complimentary, but my performer's ego wants more. It's greedy - it wants to hear how funny or moving or just plain wonderful my performance was. ;)
And the fewer people you share a curtain call with (20-person chorus? the other 2 members of the household staff? you & your chief foil?), the more likely you are to get a hand from the audience. Lee Adams had it right - "the sound that says love: applause!"