I know the common misconception is that they did defend the pass largely on their own, and that in reality they had about 10k other Greeks with them, but I wonder what the Spartans intentions were.
Did they only "go for a walk" because they knew they had help coming or was it a surprise that they werent going to have to defend the pass on their own?
You may be interested in the posts of /u/Iphikrates, which address this topic in great detail, including a post & podcast combo about Thermopylae with discussion regarding the allied forces. I recall that he talks a bit more about what the Spartans intended in the podcast as well.