Introduction. Users’ behaviour studies relevance shows a constantly increasing trend. Users’ behaviour is the research focus both in libraries and information sciences and in cataloguing. User studies in LIS were developed along four trends: user studies, information behaviour, information practice and information experience (Gonzales-Teruel 2018). With FRBR Report in 1998 and its four user tasks definition, a major shift toward users’ centrality came in cataloguing too. Relevance of user tasks is also underlined by the Statement of International Cataloguing Principles (ICP). It confirms FRBR four functions and adds a new one: to navigate. The focus of this research is the effective interaction of users with the OPAC at the Biblioteca Civica Vincenzo Joppi”, Udine, Italy, with the aim of assessing both efficacy and efficiency of interactions, from the point of view both of the user and of the OPAC. Method: This paper draws on an original research, as data about a simple random sample of 36 readers were collected by means both of a survey by a questionnaire before and after the interactions and by OPAC transactions screen recordings. Data were collected in Udine public library, Biblioteca “Vincenzo Joppi”, Udine, Italy, between August and October 2017. Quantitative and qualitative data analyses were conducted on collected data about 130 searches (a process that ends with a positive or negative result) and 200 interactions (each of the search attempts). Results and Discussion: The contrast between use frequency and reader’s proficiency level shows that the more frequent the use, the higher the confidence level felt by reader. On the contrast, confidence level is not related to educational background. Nearly half of the interviewed (49%) search in the catalogue to fulfil personal information needs; any other information needs (teacher suggestions, friends’ advices, internet etc.) are less important. In nearly half of the interactions, the search field used is title; instead, percentage of searches by subject, by author’s surname or by author’s name and surname are all just slightly over 10%. Furthermore, data about search field support data about the two broad types of questions: 60% are known item searches, and just 40% are exploratory searches. The most preferred search approach is Google-like search (36%); advanced search (30%) is relevant, while basic search (12%) is less important. However, the most successful results are obtained by advanced search (67%), followed by basic search (56%) and google-like search (53%). An interesting difference emerges from the contrast between data collected from questionnaires and from direct observations of readers’ interactions with the OPAC: readers express approval or disapproval based on the final results of their overall research and not on single interactions, and take for granted that to achieve results one or more attempts could be needed. Final success depends mostly upon the kind of users’ starting question: known item searches are more successful than exploratory searches (60% versus 43%). Data As a result, data confirm that to perform exploratory searches readers need to be supported by a higher level of reference service. Data show that reasons for failure in searches rely on readers’ behaviour (56%) more than on library services (24%). For this reason, readers’ interactions with the OPAC could be improved by promoting information literacy for the readers.