How do you create a new TV character?

More than 28 million people tuned in to see Ashton Kutcher (right) replace Charlie Sheen on Two And A Half Men

What do you do when the star of your TV show quits?

It's a problem that's been plaguing Hollywood this summer, with Laurence Fishburne (CSI), Steve Carell (The US Office) and Charlie Sheen (Two And A Half Men) all leaving hit shows, for a variety of reasons.

All of the programmes have opted to continue, with new characters stepping in to fill the void.

But writing a new actor into an established series creates a peculiar set of problems for the writers.

How can the new recruit be distinct without destroying the show's premise? How will the cast dynamic change? How will audiences react?

As controller of the drama production at the BBC, John Yorke has first-hand experience of placing new characters into series like EastEnders, Casualty, Doctors and Holby City.

He shared his "golden rules" for writers grappling with the problem.

Make a memorable debut

The Slater family were deliberately created to "cause havoc" in Albert Square

I was always very fond of the way we brought the Slater family into EastEnders, although there is a caveat - which is that the audience weren't!

It's really fascinating in a show like EastEnders, where the regular cast are the viewers' family by proxy. When a group of people you don't know suddenly come in and start shouting at people you do know, immediately your hackles rise. You think, "who are these people who've invaded my living room?"

But that can be a good thing. It doesn't seem like a good thing, but what it does is create intrigue immediately. Almost every new EastEnders character is hated for a month and then, you know, 60% of them go on to become loved.

Know your character's purpose

Spooks character Harry Pearce, played by Peter Firth, has a specific role to fulfil in the drama series

If you just create a carbon copy of a character that's gone before, you're going to be in terrible trouble.

One of the most brilliant examples of creating a new role is how they re-cast the colonel in M*A*S*H [actor Harry Morgan replaced McLean Stevenson, who left after the third series].

In the early years, they had someone very ineffectual and useless in charge. When he left, at a time when the show was enormously popular, his replacement seemed to be the complete opposite - a very brutal, almost dictatorial character. But essentially they fulfilled the same function in very different ways.

So you have to know the character's function within the narrative of the show.

For example, if Harry left Spooks - which he hasn't done - you know that the function of any replacement character has got to be to corral that disparate group of elements together to fight the enemy.

Keep it mysterious

A really good writer will say "sod the exposition" and remove it altogether.

Good writers know you don't need to explain who someone is, you get to know them through what they do. Now, you'll need a little bit of a story to explain why they've arrived - but a really good writer will make it a mystery.

Mystery is what keeps you watching. It's the prime narrative motor of most drama.

Be cautious about re-casting a beloved character

An entire season of Dallas was explained away as being Pam Ewing's dream

When Bobby Ewing came out of the shower in Dallas, the producers were thinking "we need Bobby back for the show to be successful". They explained an entire season of Dallas away as a dream, and the audience thought, "you think I'm an idiot, don't you?". It's hugely dangerous.

On EastEnders, we'd never recast Michelle Fowler, even though it's unlikely that Suzanne Tully will ever want to come back, because, as far as we're concerned, Sue Tully is Michelle. I don't think we'd ever bring Cindy Beale back either. First of all, she's dead, but also Michelle Collins played her fantastically.

With some of the lesser-known characters, or ones who haven't been on screen for 10 or 15 years, you can sometimes do it. Normally we send them up the stairs for a couple of months and they come back different.

But it's a dangerous road to go down, because the audience are on some level thinking, "why are you lying to me?" It breaks that bond of trust.

Don't stray too far from the show's premise

As with M*A*S*H, if you're replacing a character, a good trick is to make them appear different to the previous person but, underlying that, they do exactly the same thing.

Spooks is a rather brilliant example. Over 10 years, it has continually managed to re-cast itself with characters that look very different but who are finally revealed to have very similar functions.

Most shows die after three years because you run out of stories. But by introducing new characters with new stories, you get another three years life expectancy. You've just got to get it right. Skins is another programme that's proved that you can very successfully re-cast each year.

Listen to your audience… but have faith in your ideas

You'd be foolish to ignore the viewers, but you'd be just as foolish to instantly react when they're unhappy.

The great thing about a show like EastEnders is that you have time to adjust and tweak as you listen to feedback. It was very interesting with the Slater family that everyone on the show felt very strongly from before we started filming that we had something very special.

It was one of those rare occasions where a really good cast bounce off each other. There were four or five months of fairly virulent abuse. Then it calmed down until, eventually, they became some of the most-loved characters of all.

The turning point was when the audience discovered that Kat was Zoe's mum, because it showed she was vulnerable. Everyone thought she was this loud-mouthed and gobby person, when in fact she was caring and selfless.

This was long before Zoe found out, which took about another year, but about three months in we let the audience in on this secret, and that changed everything.