A summary should be accurate and while 100% objectivity isn't possible, the summary writer should strive to stay as close as possible to this position. Most importantly, the summary writer should fairly represent the author's ideas. Writers of summaries should save their own ideas and interpretations for the response, rather than including these things in the summary.
The purpose for the summary can alter how it is written. Also, the reader's needs and interests must be considered when crafting a summary. A key skill to develop for use in written summary is the ability to paraphrase (to express the author's ideas using the summarizer's own words).