'All in good time,' said a cagey Lorne Michaels, longtime executive producer of NBC's Saturday Night Live, which has been rejuvenated this fall by Fey's three skits as the Republican vice-presidential candidate.

Mr Michaels said on Wednesday he wasn't actively seeking Mrs Palin, but that the McCain campaign called after the first skit, when Fey's Palin appeared with Amy Poehler's Hillary Clinton on the show's Sept 13 season premiere, to say they enjoyed it.

Saturday Night Live has a long history of political walk-ons.

Mr Michaels prefers keeping this sort of news a surprise until it happens, an opinion reinforced when word leaked that Mr Barack Obama would be on that same show and the Democratic presidential candidate had to cancel at the last minute. 'I think we looked stupid,' he said.

There are three more first-run Saturday Night Live episodes before the election. Starting on Thursday, NBC is also airing three prime-time editions of the show at 9.30pm EDT (9.30am Singapore time).

Mrs Palin told reporters on Tuesday she'd love to appear on the show with Fey.

'I love her, she's a hoot and she's so talented,' Mrs Palin said. 'It would be fun to meet her, imitate her and keep on giving her new material.'

From the moment Mrs Palin was selected as Mr John McCain's running mate, Mr Michaels said he barely had time to consider the idea of Fey impersonating her. Others did it for him.

'The next day the doorman in my building said, 'What a gift, you're going to have so much fun with Tina Fey,'' he said.

Fey needed some convincing, primarily because she was busy with her Emmy Award-winning role as harried late-night show producer on 30 Rock. The day of SNL's season premiere, she was shooting an episode of 30 Rock with Oprah Winfrey as guest.

'There are certainly people here who could have played her and played her well,' Mr Michaels said. 'But the audience would have been disappointed if it had not been Tina. They cast her.'

During that first impersonation, Fey got laughs simply by nailing Mrs Palin's accent. She described global warming as 'just God hugging us closer'. Mr Michaels knew he wanted Fey back for the Oct 4 show, two days after the vice-presidential debate. But Mrs Palin's interview with CBS' Katie Couric was so priceless, they had to write a sketch around that, he said.

In one answer to a question by Couric, played by Poehler, Fey gives a circular response of campaign cliches that reaches a dead end. Asked for specifics on how a McCain administration would spread democracy, Fey's Palin said, 'Katie, I'd like to use one of my lifelines.'

Through the first three weeks of the season, Saturday Night Live has averaged 8.3 million viewers, or 49 per cent more than last year, according to Nielsen Media Research. The skits have also drawn tremendous web traffic, with 9.3 million people watching an online clip of the 'Clinton-Palin' segment. The 'Couric interview' has been seen by nearly 7 million people, NBC said.

'She's made 'Saturday Night Live' look, for the first time in a long time, like it's playing in the same satire league as Comedy Central,' said Mr Robert Thompson, director of the Bleier Centre for Television and Popular Culture at Syracuse University.

Fey has been off Saturday Night Live for two years, but was the guest host last winter in the first show back from the writers strike, where she made a vivid defense of Hillary Clinton.

'With all her years on 'Weekend Update' and even more as Liz Lemon on '30 Rock,' she's become someone the audience trusts,' Mr Michaels said. 'She's credible. And I think none of that would have mattered if her take on Sarah Palin hadn't been fresh and funny.'

While the comic impersonation is tough, Fey's character is likable, much like Will Ferrell was in his days talking about 'strategery' as Mr George W. Bush, he said.

Richard Greene, a public speaking coach and author of 'Words That Shook the World: 100 Years of Unforgettable Speeches and Events,' said if he were a Democratic official, he'd be pulling any favour he could to keep Mrs Palin off Saturday Night Live.

'She is so charming and so media savvy,' Greene said. 'When she has a script, she will completely seduce America.'

Mr Michaels is enjoying the ride, letting Fey know that she only has to impersonate Mrs Palin through Nov 4.

But what if she is elected the next vice-president?

'I think we'll find somebody else to play Sarah Palin,' he said. 'I don't think she's going to be playing Sarah Palin for the rest of her life.' -- AP