Place it just after the confusing line or reveal, as the first reaction in the sequence, then follow it with Ceeday Huh for a second reaction, or with Oh My God Wow when the moment escalates further.
Über diesen Sound
This is a very short clip of Markiplier saying 'what' in a flat, caught-off-guard voice that stretches the single word into a clear double-take. It feels confused and slightly suspicious, like someone has just heard something that does not add up. The clipped delivery makes the disbelief land instantly, without any setup or explanation.