The character was seemingly killed off in season seven, but will come back in Friday's (March 23) episode suffering from amnesia.
Teasing his reappearance, Collins explained that Castiel will find it difficult to trust his ex-accomplice when they meet once again.
"At first when Cas returns, he has a little bit of wariness of Dean," the actor told Zap2It. "He can tell that Dean's a dangerous guy.
"Cas is not quite positive that he's an ally, at first, and Dean is not being totally forthcoming with returned-Cas either."
Collins went on to reveal that the character, who will emerge "drenched and confused and unclothed" from a lake, will apparently revert back to his old persona.
'Honestly, it felt more like a hard reset than like a new version of the character," he hinted. "Like we were going back to Castiel at the beginning... I definitely felt like we were returning to ground that we had already tread on with Castiel."
Supernatural airs 9/8c on The CW.