Fill in the detective's notes about the highly suspicious suspect.

While the suspect was staring blankly at the interrogation room wall, I suddenly realized he was wearing a fake mustache.

While the suspect was staring blankly at the interrogation room wall, I suddenly realized he was wearing a fake mustache.

"Staring" is a continuous background action, so it takes the past progressive (was staring).

"Realize" is a stative verb (a verb of thought or mental state). Stative verbs generally cannot be used in the progressive tense, so it must be in the past simple (realized), even if it happens during another action.