Michael Mayer and I are sitting perhaps a tad too close to each other. We are in a tiny hospitality room behind the royal box at the Savoy Theatre where the Menier Production of Funny Girl has transferred. The walls are gilded with gold paint and the wallpaper is pure art deco opulence. And then the walls start closing in. Or they feel as if they do because Michael Mayer, one of the hottest musical directors around and a man of powerful bearing and generous girth, is not enjoying my line of questioning. Eventually he lets rip.

"Can someone other than Barbra Streisand play this part? Yes! Can someone other than Barbra Streisand give you a reasonable version of Fanny Brice? YES!" He's practically shouting now. "Can someone who doesn't look like Fanny Brice play this role really well? YES! Can someone who doesn't sound exactly like Fanny Brice still give you Fanny Brice? Absolutely!"

We are talking - sparring - about Sheridan Smith's version of the New York, Jewish star of musical theatre, Fanny Brice, who was immortalised by an even bigger New York Jewish star Barbra Streisand. Streisand's performance on stage, and especially on screen, is so iconic, it's as if Brice and Streisand are as one in the public imagination.

Smith, it should be said, is terrific. She brings a wonderful comic instinct to the role. However, you could be forgiven for thinking the West End's casting stars have misaligned because if there is one performer currently out there who would be perfect as Fanny Brice it would be Samantha Spiro who has played the role before but is now playing Miss Adelaide in Guys and Dolls, and if there is one person in the West End of whom it might be said she would be a perfect Miss Adelaide, it's Sheridan Smith.

Anyway, her funny girl is very funny. Sure, the critic of at least one paper - not this one - suggested that Smith lacks a certain Jewishness. But everyone seems to think she's great. And even if Sheridan is not as quite as kosher as some may like, with Jews such as Mayer directing and a script revised by Harvey Fierstein, the show is briming with authenticity. "Harvey has helped keep it honest," says Mayer.

Still, Streisand's reputation in the role is a helluva thing to live up to. So maybe there was a bit of pressure building in advance of the opening night, which was last Wednesday.

But I didn't ask about Streisand or about Smith's Jewishness. I asked whether a role such as Fanny Brice is so big in musical theatre, that it's necessary to go back to the original real life Fanny in order to perform the role. Maybe it's not the most brilliant opener but, you know, it should get a conversation going.

"I would think so," is Mayer's response, delivered a little archly. And so it went on - me trying to justify the question, Mayer attempting to kill it at birth: "What are you asking? How do you do Funny Girl without doing a Streisand impression?"

"Well, that's an element of it…" I stutter, before repeating the question, only less clearly.

"You're annoyed by it, aren't you," I say perceptively.

"I am, because it feels like you're hedging. It doesn't feel like an honest question. It feels like the question that everyone…"

"Everyone wants to ask?" I want to say. But there's no time.

"It's just that this Streisand thing hovers over it," continues Mayer "And any question that's circuitous feels disingenuous."

Smith is not trying to sing like Streisand he explains. Part of the brilliance of casting her is that she couldn't.

"She is a wonderful singer, but she's a singing actress." This is true. And the fact that the award-winning star is not Jewish is irrelevant, he says. "I don't care about that. That's acting. That's an uninteresting question to me, right off the bat."

Perhaps Mayer is feeling particularly protective of Smith. Both have a sick parent: Smith had to bow out for some of the show's Menier run when her father was diagnosed with cancer. And Mayer is intending to pop back to Washington where his mother Louise lives, for Pesach. She's dealing with two terminal cancers. "It's not fun," he says. She won't have to cook. He's ordered an entire Passover meal as a kind of takeaway to be delivered, which apparently you can do in America.

It was Louise who outed her son, he once told me when we met previously. She knew he was gay before he had said anything to her and, one day when they were doing the washing-up together and Mayer was telling her about another break-up with another girlfriend, she said "Honey, don't you think you're gay?"

It saved her son a lot of anxiety about when and how to come out.

The conversation is coasting now. On his Jewishness, Mayer describes himself as completely secular. "A bad Jew" in terms of religious observance.

But he's what some Jews would consider good in the most important ways. He's a lover of Israel -– the liberal Israel of Tel Aviv - and identifies as "a humanist" kind of Jew.

He was there, in Israel, with husband Roger, a doctor, just before rehearsals started in the West End. And somehow it felt right moving from that world into the Jewish, Brooklyn-ish setting of Funny Girl.

Which brings us back to Streisand. She is reported to be nearing production of a long-planned film version of the musical Gypsy. Mayer, who has just finished a film version of Chekhov's The Seagull, would be a perfect choice to
direct it.

"I'll be really honest," he says - not that he's been anything but so far. "They sent me the screenplay by Julian Fellowes (who wrote Downton Abbey). But it wasn't for me. His take on it lacked a kind of authenticity. I wouldn't be at all surprised if they brought on another screenwriter. It didn't feel like Gypsy to me. It felt English," he says candidly.

The mood is almost warm. He apologises for his display of "attitude…

"If the show does well and continues to New York, I'll still be" - he rolls his eyes - "at the Streisand questions. But I'll have the answers all prepared."

Yet Streisand's Fanny Brice will loom even larger over there, I say. The public's perception of the role is surely even more indelibly linked to Streisand. Hackles rise, then fall.

"I don't do this for public perception," says Mayer. "I do it because I want to tell the story the best way I can given the recourses I have. And I was blessed to have such a resource as Sheridan Smith.

If Sheridan Smith wanted to play Idi Amin in the musical version of his life story, I would have no trouble casting her."