The "talking and coding" point mentioned above is key. The interviewers want to hear your thought process and approach. Gayle McDowell (author of CtCI) has some videos for purchase[0] that show mock interviews including whiteboard coding and she gives good feedback. Certainly worth the $40 if you are unfamiliar with this style of interviewing and going for a dream job.

[0]: https://www.careercup.com/video