Kristen Bell will appear in the 'Gossip Girl' finale.

The 'Forgetting Sarah Marshall' actress - who recently revealed she is expecting her first child with her fiance Dax Shepard, 37 - voices the villainous titular character and although she has never appeared on screen in the show before, TV Line reports she will be featured in the December 17 hour long special.

On Friday (09.11.12), she tweeted: ''The saddest day I've had in a while...Last day on #gossipgirl Ill miss you, you snarky bitch!''

Kristen got engaged in to Dax - her co-star in 'When in Rome' and 'Hit and Run' in 2009 - after dating for two years.

Earlier this year, they revealed they were planning to start a family, but have previously said they don't want to tie the knot until gay marriage was legal in the state of California, where they live.

In February, Kristen, 32, said: ''The reason we're not rushing to get married is because I don't feel appropriate taking advantage of a right that's denied to my [gay and lesbian] friends.

''That's why we've been so hesitant. Dax and I have talked about it a lot, and this issue is very important to both of us. We're just standing up for what we believe in.''