You could start by finding some synonyms for "duty" and "self-interest". Having a range of terms for these key words will be handy for your essay writing. Then, consider how these two ideas are represented in the two texts - don't forget to think about how minor characters might be used to convey these ideas too. If you look at "duty" first, in the film this takes different forms - Mandela's commitment to his country, his obligations to his family, the loyalty of his security guards, Pienaar's development of his leadership style as an expression of his devotion to his team members. In "Ransom" you could "match" these with Achilles disenchanment with serving Agamemnon, Patroclus' willingness to repalace Achilles on the battlefield, Priam's reminder that Achilles is also a father and the way he fosters empathy in him.