At most job interviews, it is expected that you will demonstrate your teaching by taking a class. Anyone can have a perfect application and lots of people are good at interview, but teaching is what they'll be getting paid for.
Teaching as part of the interview is a good way for you to suss out the school too. How do the kids respond? Do they look interested and on best behaviour for a visitor or are they uninterested and rowdy, despite the presence of the head at the back of the room?
The interviewers will want to see that you can teach: snazzy activities and resources are a bonus, but focus on the basics. Read the person specification and try to give them that. Think about how you can show that you're professional, have a rapport with children and manage them well, are enthusiastic, plan well, use effective teaching strategies, and reflect on learning and teaching.
Plan a snappy starter that assesses current levels of understanding; have clear learning objectives; a main part that teaches them something new or helps them revise their learning; and then a plenary that enables you to assess their progress.
Appearance is important. You've got to feel good and look the part in the interview and in the classroom. Wear smart clothes, but ones you'll be comfortable teaching in.
Schools vary in how much guidance they give you on what to teach. Many give you a completely free rein, or are pretty vague ("something for a literacy lesson"). Others are more specific, such as "Year 7 mixed ability French: recap hobbies and the present tense", or "Year 9: a 50-minute lesson on recipe development for a pasta dish".
There are a million ways to teach anything and though you want to look interesting it's probably not good to do anything too adventurous while being observed. It's better to teach a standard lesson well than have chaos. Keep the lesson simple.
Give the interviewers a computer-processed copy of your plan - check for spelling errors. Make sure you have a plainly phrased learning objective and some motivating activities that will allow the kids to meet it.
Bring your own resources. Find out the name of a child who is very able and one with special needs (any more than two and you'll forget). Think of questions that will be appropriate for each. Make lots of eye contact with the pupils, smile, and use praise to reinforce the kind of behaviour you want.
Afterwards, reflect on the lesson honestly and intelligently showing that you can assess pupils' answers, and think of ways to improve your teaching.
No one expects you to be perfect, but your interviewers want to see that you're enthusiastic, and reflective.
Be modest when it goes well.