I have been a visual merchandiser at Topshop, Mango and for Mary Portas' Creative Marketing Agency so my experience is a bit mixed but since I mainly get asked about Topshop, here's a quick guide of some of the basics(I'll probably add to this as time goes on as this was done in haste).
At your interview they will most likely have you do the following 3 things:
1. Style a mannequin to one of the current Topshop trends which they will tell you. STUDY STUDY STUDY the trends on the website 'til you know them off by heart. Go into a Topshop store before the day of your interview and really look at how they have styled the mannequins. You will likely be given a time limit for each task so visit the store before your interview and really look at where the trends are in the store as this will help you save time when it comes to styling them.
2. VM an area. Again they'll give you a trend (different from the previous one) and probably a few empty different types of fixture/rails. They'll give you a bunch of clothes on the rail and again prob give you 20 minutes to sift through the product to select the best product for that trend and style the rails accordingly. The way I always used to try do it would be to style the first rail in a strong colourway, have the second a step away from the previous colour way but perhaps still using one of the main colours from the first and have the third a different colourway altogether but again perhaps using one of the key colours from the second. I'll give you an example:
Fixture 1 - BrightPink SeaGreen Black White
Fixture 2 - BrightPink DarkBlue Black Cream
Fixture 3 - PalePink Cream Black
So basically keep strong colours together. e.g. Cream and White should be kept seperate. In this case, even though there might be 2 pinks in the trend, since one is BrightPink and the other is Pale, they are kept to seperate fixtures.
On one arm, try to think about an outfit build. The product at the front (in customer's eye line) should be a key trend piece so say it was a dress, you might want to add leggings or a jacket behind it but always be aware of everything a customer sees in their key line of vision should look great. ie. don't put plain black leggings at the front of a rail.
You'll also be expected to add accessories to these rails. Keep it simple but add at least one prong arm to each fixture.
Colour balance is one of the key things to remember, don't over saturate a colour. Spread it around the fixture (or wall) so that from all angles the trend looks strong.
3. Interview. Topshop mainly look for 2 things in their employees: People who have personality and people who've got the customer in mind. Let your personality come out. If you are creative, let them know that but always bear in mind that the aim is to create a great customer experience and help them see how the trend translates into something wearable for them (and therefore commercial for the store). In Topshop they regularly look at key products performance to make sure they put these products in key sightlines for the customer. Be cheery and happy and keen to learn and show what you can do and they'll love you.