Mode can be both big and small: a tiny detail like a non-fluency feature (umm...errr) or a non-standard spelling are worth looking at and relating to mode, but don't forget the bigger picture of what each mode can offer (affordances) and how it can hold you back (constraints) .
E.g. think about why a particular mode (and technology?) might have been used: what does it offer that another mode doesn't? And how does it restrict what you can say or write?
Mode is for life, not just for Christmas: don't just talk about mode in your intro and then forget about it like that unwanted Christmas puppy. Weave mode into the rest of your answer.
Channel your thoughts: don't forget the concept of channel. You've got texts using the visual channel and/or the auditory channel. Even fairly basic points about (say) graphology (in the visual channel) or emphatic stress (in the auditory channel) can become quite significant in a text when you relate them to mode.
Things mean things: don't forget meaning. One of your first jobs should be to work out the following:
- What is each text about?
- How is that topic being represented?
- What views and perspectives are being presented to the reader/other speakers?
- How can we tell?
*or any other English exam