This is juicy! Alicia Silverstone was slated to guest star on How I Met Your Mother but backed out when she was told Britney Spears would playing her assistant. Alicia was concerned Britney would overshadow her performance.
In her defense, Brit tends to over shadow anyone in her general vicinity. Producers have called in Scrubs' Sarah Chalke as Alicia's replacement, but an insider hints that the writers love Alicia so much because she's so easy to work with that they plan on creating another character for her.