Her performance here shows what a terrific actress she is. She's arguably the finest in this movie, surpassing even Ustinov. Have you seen Murder By Death? A very funny murder mystery spoof and Smith is fantastic in it again.
I love how she starts out very wary of Poirot when he arrives, then later does a 180 and is gushing with praise for him as she tries to convince him to save her hotel by solving the murder... then hints at blackmail about HIS reputation if he doesn't.
But my favorite bit is when she's trying to help, and Poirot gets annoyed when she has inadvertently eliminated his last suspect.
"Oh-- OH!! I see you what you mean. You mean... NOBODY did it!" "And yet-- we still have-- a body."