How do I become … a voiceover artist

The impish boy on the end of the phone line is superseded by a lisping little girl, before the boy's mother and then an elderly lady from somewhere up north take over. Just as an alien is clearing its throat, Joanna Ruiz reclaims the conversation.

Ruiz, 42, has supplied voices for a range of children's shows, including the Horrid Henry cartoons, and her more formal tones will be familiar to many who have endured computerised employee programmes on health and safety in the workplace. "I've just done one of those for Fife NHS on heart surgery about which I know nothing," she says. "Often I have no clue what I'm saying, but the trick is to sound as though I do, so I have to learn how to pronounce all these strange medical terms."

The skill of a voiceover artist is so singular that even established actors can struggle. "A lot of people reckon they can do a funny voice, but it's not enough – you have to bring the script off the page for an audience who can't see you," Ruiz says. "It's amazing how many famous people want to do it, but don't get offered the part because they are too used to acting with their whole body."

Ruiz enrolled at the Royal Welsh College of Music & Drama with a conventional acting career in mind. During her course she sang in a band and was introduced to a woman who sang jingles for TV adverts. "She suggested I do the same and one day the chap who ran the recording studio we used mentioned that the original soundtrack to Casper the Friendly Ghost was being re-recorded," she says. "He urged me to try out for a rabbit, so I practised and practised from the old soundtrack and ended up getting the lead part."

Ruiz discovered that she excelled at emulating small girls and decided from then on to divert her acting skills into voiceovers. "It gives you so much more scope," she says. "In Toby's Travelling Circus, for instance, I play Toby but also his mother and several other characters."

Adult female actors are frequently called upon to impersonate small boys because their voices are lighter than their male counterparts and the recruitment of children involves so much additional red tape. Ruiz, through dogged practice, has learnt to produce the right sound from the back of her throat. She is a collector of voices. "I'll often have [BBC children's channel] CBeebies on to find a new voice. I'm starting something soon in which I have to be an alien, so I'm casting about for the right sound for that."

Cartoons, unsurprisingly, are the highlight of a job that can encompass home learning videos and vacuum cleaner ads. "Those are hard because you have to sound excitedly enthused by a product you care nothing about. The trick is to smile while you're talking because that makes your voice sound bright and cheery," she says. "If I know I've got a couple of days of animation work, it feels like a holiday. You're in there with the other actors and the interaction makes the script come alive."

The secret of success in a competitive market is a strong showreel to demonstrate your range. "Listen to TV ads and practise, practise, practise," Ruiz advises. "Take a script and read it with silly voices because you have to remember to act, too. You need lots of ups and downs in your voice and more expression than in straight acting roles since the audience can't see you."

A drama qualification is, she reckons, unnecessary, for voice workshops teach every aspect of voiceovers, including specialist courses for voicing computer games – "that's less voice and more reactions like cries and yelps when you're hit with a brick or knocked off a cliff."

Agencies such as The Showreel in London will help beginners put together a professional standard voice demo as well as offering coaching and courses. "Listen to the reels of actors on the websites of voiceover agencies to get an idea," Ruiz says. "And when you've got your reel, go round production companies and approach local radio stations to see if you can do an ad."

Versatility and persistence are qualities almost as important as an accommodating larynx. "I always struggled to do old-lady voices, then found I could only manage it if I gave them northern accents," Ruiz says. She adds that a natural passion for the human sound is also a must.

So thoroughly does she embed herself in a character that voices will often come unbidden after she has left the studio. "I find, especially if I've been doing an American voice, that I'll be speaking it for a couple of hours afterwards, and my children have grown used to the many different sounds of mummy."