David Letterman apologised for not realising how depressed Robin Williams had been.

The 'Late Show' host paid tribute to the late actor for the first time since his suicide last Monday (11.08.14), showing moments from the 'Jumanji' star's ''almost 50'' appearances on the programme and admitted he was sorry he had ''no idea'' how he was feeling before his death.

He said: ''What I will add here is beyond being a very talented man, and a good friend and a gentleman, I'm sorry, like everybody else, I had no idea that the man was in pain, that the man was suffering. But what a guy, Robin Williams.''

David, 67, met Robin at The Comedy Store in Los Angeles almost four decades ago, and the presenter admitted while he was ready to mock ''the new guy'' when they first met, he was quickly won over by his undeniable talent.

He added: ''All of a sudden he comes up on the stage and - you know what it is - it's like nothing we had ever seen before, nothing we had ever imagined before.

''And then he finishes and I thought, ''Oh that's it, they're gonna have to put an end to showbusiness because what could happen after this?' Honest to God you thought, 'Holy crap, there goes my chance in showbusiness.' ''