IT has more character than Robin Williams' acting CV, it's where scores of school kids flock on a Saturday to watch subtitled world cinema for free, and where Hollywood heroes Robert De Niro and Steven Spielberg are never without a seat.

Inside the Modernist post-Art Deco foyer of the Glasgow Film Theatre, the city's oldest-remaining cinema, you're never stuck to find something of interest.

As the GFT management once again prepares to halt its regular screening schedule to allow the public a peek behind-the-scenes for Doors Open Day, its director Jaki McDougall is steeling herself for the questions that always raise a smile.

"Some people will say: 'Oh, the old Cosmo, we thought you were shut'," she says.

"Other people think we're a porn cinema - I'm not joking!

"Other people think we're a club, like members only.

"Being part of Doors Open Day reminds people that we're still here and so fabulously up-to-date."

The building has captivated audiences since it opened in May 1939 as the Cosmo Cinema, but Jaki is keen to stress that they have exciting plans to open a new chapter in its history.

As she winds through the rabbit warren of corridors to her office, she ponders plans for an extension.

"We own this part of the building, which is caught within the Littlewoods building, and we own the GFT building.

"We're looking at the possibility of building on top of the existing building, or could we dig down? Or are there any other premises that we should be looking at?

"We're always short of space to do things."

The GFT screens around 600 films a year, of which around 60% are foreign language. In the past, its red carpet has been rolled out for an A-list roster of stars, including Mickey Rooney, Martin Scorsese, Sean Connery, David Lynch, Baz Lurhmann and Tilda Swinton.

The famed seat sponsorship scheme, of which Ewan McGregor is patron, boasts the illustrious names of Steven Spielberg, Robert De Niro, Michael Caine and Orson Welles.

Cult director Quentin Tarantino also loves the independent cinema, which he visited in 2007 when he introduced Death Proof and participated in a Q&A session with city film fans.

Each year the staff work with more than 20,000 young people through over 200 education events and through the programming of its Saturday morning Take 2 free family screenings.

After a worrying dip in audiences following the opening of the 18-screen Cineworld (then UGC) multiplex in September 2001, the B-listed art-house cinema just up the road in Garnethill is enjoying a renaissance, with ticket sales up by 30%.

The success is attributed to a ripple effect caused by the Glasgow Film Festival. Launched in 2005 which is now ranked as the UK's third-most-popular festival after London and Edinburgh.

Meanwhile, in May of this year, the GFT's 45 staff blew out the candles on two birthday cakes. The first fell on May 2 when they marked 35 years since the cinema opened its doors as the Glasgow Film Theatre, after being bought by the Scottish Film Council.

The second was the 70th anniversary of the building itself.

Opened on May 18, 1939, the Cosmo was a visionary venue, the result of a collaboration between cineastes George Singleton and Glasgow Film Society founder Charles Oakley.

The building remains largely true to the pre-Second World War design of local architects James McKissack and WJ Anderson II.

The cinema underwent a major change in 1988 when the original 850-seat auditorium was split in two: the stalls of the original cinema became the 142-seat Cinema 2, and the 404-seater Cinema 1 was created from the circle.

The Cosmo's globe motif, with Glasgow at its centre, is still represented in the foyer's mosaic floor created by artist Todd Garner, and also in Cinema 2's cosmic curtains, designed by city textile duo Timorous Beasties.

Mr Cosmo, whose bowler-hatted caricature became synonymous with the cinema, lives on in the name of the cafe-bar, a cartoon image above the box office, and in the large-scale light fitting in Cinema 1. From the bar to the projections rooms, Jaki, above left, and Barney, above, take great pride in their work at the GFT

Come Sunday, 20 September, the cinema will halt normal screenings for three hours for Doors Open Day, turning two screens over to archive film screenings and an interactive 3D project with New Media Scotland.

Film fans will be able to wander around the cinema's curious nooks and crannies, taking in the powder room on the way to Cinema 1, the education room, the mezzanine bar and the projection rooms.

If you're lucky, it might mean a chin-wag with the cinema's veteran technical manager Barney McCue, who has worked at the GFT for 28 of its 35 years.

The 61-year-old, from Baillieston, born in a tenement above the New Grand Cinema in Cowcaddens, learned his craft with the old ABC cinema chain. It's his job to oversee the seamless running of all screenings.

"You've got a good variety - it's not just showing movies," says Barney, whose wife Sadie works in the cafe.

"Here you're working with directors and filmmakers, you have a lot of festivals, you have young people making movies, and we also do satellite link-ups."

Neil Baxter, of the Royal Incorporation of Architects in Scotland, will give a lecture entitled, The Cinemas of Cinema City - from the Hillhead Salon to the Cosmo at 2pm on Doors Open Day, in which he'll explore the city's love affair with the flicks.

The GFT and the city's multi-storey Cineworld have now established a reciprocal relationship, working together for the Glasgow Film Festival and Take 2 events.

Cinema director Jaki, born in the Gorbals, raised in Priesthill, and who came to the GFT 10 years ago, says proudly: "Our place just sparkles. All the brass is hand-polished, the carpets are clean, there's always ushers in the cinema - you don't get that everywhere." The GFT, 12 Rose Street, will be open from 11am until 2pm on Sunday, September 20 as part of Doors Open Day. Free tickets for Neil Baxter's talk and for projection box tours will be available on the day on a first-come first-served basis. For info: 0141 332 6535.