After his old band opened the Children In Need gig with 'Greatest Day', Williams performed 'Bodies' and 'You Know Me' at and later rejoined the group for an all-star rendition of The Beatles' 'Hey Jude'.
Williams told The Sun: "I was tearing up before I came on, knowing what was just about to happen and what the reaction was going to be like.
"We should have milked it for a lot longer. I was trying to call them back and do a bow. It's the first time in 15 years we were together, we should have at least stayed for 15 minutes."
He added: "While they were singing I wanted to grab a mic and start singing too. It would have to have been 'Back For Good'. I'm not nervous around them anymore because we've spent a lot of time around each other now.
"All the bitterness and resentment has gone. We're just left with four lovely lads who I love to pieces. It's total respect now."
Of a future reunion, he confirmed: "The ball is definitely rolling. We don't know how to do it yet, nothing is planned but I'll be keeping the Brits to myself. We'll save it for something special."