A great way to show that you are studious is to include sources that were not assigned. These should be appropriate to the level of analysis that you are expected to provide. See above on how to show that you actually read these other sources. And once again, follow the directions: some assignments require additional readings and ask you to use those readings in particular ways; other assignments require that all the ideas be your own, strongly recommending that you do not seek information elsewhere.
Remember that it is better to put quotation marks around a big block of text than to plagiarize and possibly get caught. If you must use a big block of text, break it into smaller pieces: "Smith says that 'xxx.' He adds, 'yyy.' To conclude this part of the argument, he points out that 'zzz.'" You also could add a few paraphrases along the way. Effective use of these techniques shows that you are analyzing the source as you go, rather than simply quoting it.
Note that sources must be cited properly (if your instructor or area of study requires a certain citation style) or at least consistently.