If you don’t get unlimited data on the iPhone 5, it’s better to download movies onto computer and then transfer them to iPhone 5 for viewing.
How to sync movies from PC to iPhone 5
1. Download and install latest version of iTunes.
2. Connect iPhone 5 to your computer with Lightning to micro USB Cable, and open iTunes (if it doesn’t open automatically).
3. In iTunes, select your iPhone name under DEVICES in the sidebar.
4. To sync iPhone 5 with iTunes library automatically, simply press “Sync”. In this way everything in your iTunes will be synced with iPhone, including Audio content (music, podcasts, audiobooks, and iTunes U content), Movies and TV shows, Photos, Books, Contacts, Calendars, Documents (File Sharing apps only), Applications, Bookmarks, etc.
5. To sync music and movies manually, check “Manually manage music and videos” in Options area and click Apply. This will deselect the automatic sync settings for the Music, Movies, and TV Shows tabs. Then you can drag and drop movies from iTunes Movie library to the Movies tab under your iPhone 5 in the sidebar.
Tip: Manually sync could be a workaround for those who want to copy some music/videos from a new computer without affecting the data on your iPhone, since iTunes will overwrite everything on your device to keep it exactly the same with computer under auto sync mode.
It is recommended that you sync your iPhone 5 with iTunes at least once a week. In the unfortunate event that data on your iPhone is lost, this will ensure that you have a recent backup file to restore with.
What kinds of videos/movies can be synced to iPhone 5
iTunes can sync your music, movies, TV shows, podcasts, and iTunes U content to an iPhone, iPad, or iPod. iPhone 5 now supports up to 1080p HD movie purchases and rentals. If you plan to use the iPhone 5 to watch movies, you’d better get 32GB or 64GB model, since a 2 hour SD movie is about 1.5 GB, and a 2 hour HD movie is about 4 GB.
If you’d like to sync self-made films to the device, always check if they are in supported formats by iPhone 5. According to Apple, MKV and AVI videos are not accepted formats and you have to convert them to iPhone 5 optimized H.264 .mp4 videos before you can add them to iTunes.
iPhone 5 Supported Video Formats
How to convert MKV/AVI/M2TS/TiVo videos to iPhone 5 supported 1080p movies