Q: Can I use the same disk for Time Machine and as a central store for movies etc i.e. partition it ?.

Yes. But you need to partiton the drive into at least two part (one of them will be Time Machine).

Plug it in and use Disk Utility to format and partition it. Name the patitions.

Turn on Time Machine and point it at one of the partitions. Use the other for storage.

It is dangerous to place live files on your backup disk.  If and when that disk crashes you will lose those live files.  They need to be backed up on a separate backup disk.

Purchase two hard drives: one for Time Machine and one for to offload your live files.  The Time Machine drive should be larger than your offload disk plus your system disk because Time Machine is going to backup both of them.

Format the drives as Mac OS Extended (journaled) and plug them in.

Then  > System Preferences > Time Machine > Options… > Remove the offload HD name from the exclusions list.

Now both your system disk and your external offload disk will be backed up onto your Time Machine disk.

For more about backups:

Time Machine Basics: http://support.apple.com/kb/ht1427

Most commonly used backup methods: 

https://discussions.apple.com/docs/DOC-3045

Methodology to protect your data.  Backups vs. Archives.  Long-term data protection:

https://discussions.apple.com/docs/DOC-6031

PlotinusVeritas gives some great suggestions for purchasing external hard drives in this thread:

https://discussions.apple.com/thread/5602141?tstart=0