Seth Rogen proposed to his girlfriend while her "boobs were out".

The 'Knocked Up' star was so nervous after he bought the engagement ring he popped the question to his long-term girlfriend Lauren Miller almost immediately after buying it and while she was getting dressed.

During an appearance on the debut episode of Conan O'Brien's new TV show, he explained: "She was in our closet changing, and she was literally only in her underpants, and her boobs were out. I'd already kind of started, and I was like 'Oh man, her boobs are out! That's not part of the plan!' I didn't picture it like this, and I know she didn't picture it like this. No little girl is like, 'It'll happen in a closet with my t**s out.'"

But Seth - who has been dating Lauren since 2004 - had already gone too far and so he continued with his proposal because he "figured the only appropriate thing was to place the ring on her nipple".

Seth insists trying to keep the purchase of an engagement ring secret is as nerve-wracking as trying to keep a "truckload of heroin" under wraps.

The 28-year-old actor - who got engaged in September - said: "Literally, I felt like someone had given me like a truckload of heroin to hold onto. I felt like the feds were going to kick in my door at any second. I couldn't have a conversation with my girlfriend. All I could think of was this ring. Like, 'Don't say 'Lord Of The Rings!' Don't mention anything about a ring!"