epistolary diary exercise

It is less that a month to go before our long assignment is due- out fictional diary of a castaway. I have changed my mind on this more times that I can count but i think i have now settled on a final course. I have found plotting a time line helps- deciding where at the various points (1 day, 3 months, 6 month) etc various developments will occur. Having the dates prescribed naturally reduce the burden of pace and progress as we have a skeleton on which to hang the flesh of the story.
The nature of the exercise does however have other challenges. The fact that diaries are written in the first person means that the character and their backstory needs to be well developed in the head of the writer before the first word is written. Who is this person? I have struggled to decide on the backstory of my diary writer and it is a strange archaeology. How did they get to where they are now? what are their motivations? I feel in order to have the voice of the diarist be its own voice, and not simply mine, I need to develop an entire backstory on the writer/castaway. The same applies to any other characters that might populate the story. 2500 words does not really give much scope for extraneous detail and i feel quite daunted by trying to give a sense of a fully rounded person within the restraints of the specific genre and the given the length of the project. I have also found that the more research I do on geography, era, clothes, weather etc the more the story becomes real to me. For example, when i try to work through what my castaway is wearing, this immediately alters how they walk and sit and move- how straight their back, or easy their stride. I am finding that the notes I take for purely research purposes have value that goes well beyond pure information. I would really like to hear how everyone else is getting on with the assignment....