To watch an Apple TV film on a Samsung TV (which doesn't have the app) do i need an apple tv device?

For TV's that don't support the app it would require using an external device. If the TV still supported airplay then the other option would be to cast from another Apple device to the TV using that protocol. You can check out the link to see the full list of supported devices.

Apple TV app - Devices - Apple (CA)

If you are referring to the KU model then that wouldn’t be supported. Any Tv that is supported would have it available to download from its App Store. As above, the other option is to use an external streaming device.

If the Samsung television is at all recent, I'd be surprised if it doesn't have an app. What's the model?

The Samsung Model No. is UN65KK7000. I can watch Apple TV on my Samsung TV, but I have to connect my computer. Can I simply add the App to my TV?