act dumb

act dumb AND play dumb
(pretend like you don’t know something that you do know) — прикинуться, что ничего не знаешь; сделать вид, что ничего не знаешь/ ничего не происходит; притвориться, что не понимаешь, о чём речь; прикинуться идиотом; ~ прикинуться шлангом; ~ косить под дурака; ~ включать дурака

Example 2: Don’t say anything – just act/play dumb (= pretend to be stupid or not to know something).

Example: If the police question you, act dumb (= pretend you do not know anything) .

Examples from

..... The police asked where the criminals went, but their friends played dumb and said they didn’t know.

..... She’s really smart, but she thinks guys won’t like her if they know that, so she plays dumb.

..... When she asked if I knew it was her birthday, I played dumb. I didn’t want to let her know I was planning a surprise party.

валять дурака