What kind of details should be given in narrative reports of an EU funded grant project?

In accordance with Article 2 of General Conditions, reports should;

• cover the project as a whole, regardless of which part of the project is financed by EU,
• provide a full account of all aspects of the project’s implementation for the period covered,
• the period covered by the narrative report must match with the period covered by the financial report,
• include an updated logical framework matrix (LFM). LFM will be updated for reporting purpose by adding actual values of indicators into the “current value” column for each relevant level of the chain of the results. Any necessary update/modification of the targets, baselines, sources of verification related to these indicators should be made in the same LFM with due consideration to the rules for amendments. 
• demonstrate and explain the deviations that may have occurred (possibly subject to an amendment in accordance with Article 9) as compared to the initial proposal with regards to results and to means and costs.
• Have “Focus on results” approach should be kept in mind when preparing the reports, in order to provide information on the type of costs incurred and to understand their relation with the results/activities of the project. For example; to justify significant amounts featuring in the financial report, to demonstrate to fulfilment of the conditions for reimbursement in case of simplified cost options.

An explanatory notes on logical framework can be found here.