Sebastian, played by Grant Gustin, has already made clear that he is attracted to Blaine (Darren Criss).
Colfer, who stars in the show as Blaine's boyfriend Kurt, has now warned that Sebastian will take his attraction further in the next episode of the show.
"It's the peak of the Sebastian arc, and I can say Sebastian takes things to a very harmful extreme," he told TV Guide.
"It's when you see the difference between myself and Kurt. Like, if someone had done what Sebastian is gonna do in my world? They'd come up missing."
Colfer added: "I wouldn't be diplomatic like Kurt is. Kurt is way too diplomatic. I'd go get my shovel out of the back seat of my car and I'd take care of business, Clovis-style. No messing around."
Meanwhile, cast member Kevin McHale described Sebastian as a "f**king tyrant", adding: "The whole Kurt and Blaine thing really p*sses Sebastian off, and he takes it out on New Directions. So we all team up to do something about it, and Artie doesn't really agree with how Mr Schue wants to handle things."
Gustin, who recently promised that Sebastian will clash with Santana (Naya Rivera), is rumoured to be close to becoming a series regular in Glee's fourth season.
Glee airs on Tuesdays at 8/7c on Fox.
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Watch a special teaser for the next episode of Glee below: