Use of 'I'
The injection of one's own thoughts distinguishes response from summary. Remember that with summary you remain true, or close, to the original text. As such you remind your reader over and over (through author tags, for instance) that the ideas you are representing are not your own but belong to someone else.
When summary ends and response begins, you should provide some clear markers. One clear way to do this is with a paragraph change. Additionally, it is a good idea to declare your presence through use of the first person "I," as in "I believe," "I think," "I have found." Do not be shy about this; despite the familiar advice to keep your academic writing away from the first person, there are occasions where first person is not only acceptable but desirable. A response essay can be one such occasion. If, however, you are nervous about this, the best recommendation is always to check with the individual instructor for whom the response is being completed.