"In the rain, the pavement shines like silver
All the lights are misty in the river
In the darkness, the trees are full of starlight
And all I see is him and me forever and forever."
I've seen several people as both ensemble and their principal understudy track, I'm not even going to bother attempting to list them all.
I can think of a few cases where I've seen the same performer play two roles in the same show ...
Jamie Tyler as Eugene and Vince Fontaine/Teen Angel in Grease. I don't think you can get two more different roles - at least, not in Grease, heh. He was an adorable Eugene, and his Beauty School Dropout stole the show for me.
Bob Harms as Chuck and Ren in Footloose. He was first cover Ren and second cover Chuck - I never saw him in his regular ensemble role.
Daniella Gibb as Lisa and Sophie in Mamma Mia. I saw Daniella twice as Sophie on the international tour last year, when the principal was off for a few weeks with the flu. They only put notices up in the box office though, so I didn't actually discover until two days after the second show that I'd seen Daniella.
Jamie Sampson as Eddie and Sky in Mamma Mia - same tour, but this time I did know I was seeing Jamie, since I saw him the second time I saw the show, so I'd already seen him as Eddie, and Michael Xavier as principal Sky.
Claire Parrish as Columbia and Magenta in the current UK tour of The Rocky Horror Show. I saw her as Columbia in October when she was a Phantom and u/s to all three female leads. She took over the role of Magenta in December, and I saw her in that role at the beginning of February. Very different in each role, but very good in both.
Lewis Greenslade as Nick and Schlomo in the current UK tour of Fame. He's a swing and u/s to several principals. I saw him as Nick on the first night in Edinburgh, then Schlomo a few days later. I preferred his Nick to his Schlomo, but I preferred his Nick to the principal's Nick, too.
Deborah Henry: Cassie and Val in ACL (original production) Kay Cole: Maggie and Diana in ACL (original production) Angelique Ilo: Cassie and Judy in ACL (original production)