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Surface roughness affects the performance and service life of the formed components. To this end, efforts have been spent in the innovative incremental sheet forming (ISF) process. However, the researchers yet have not reached any agreement regarding the process effects on roughness. This demands further research to comprehend the knowledge in order to acquire a threshold level of understanding on surface roughening in the process. The present study investigates the processing effects upon the roughness of interior surfaces and on the friction indicator at the tool/sheet interface during forming of an aluminum sheet. Some of the sparsely studied parameters, namely flow stress, sheet thickness, and forming angle, are undertaken besides commonly considered ones (i.e., tool diameter and step size). The analysis of the results reveals that the friction indicator and roughness are analogous in respect of their response to parameter variations. Further, the roughness in general linearly increases with an increase in the friction indicator because the sheet abrasion correspondingly increases as revealed by the SEM-based surface morphology. A combination of parameters, among several attempted ones, namely dsinθ/2pt (where d is the tool diameter, θ is the forming angle, p is the step size, and t is the sheet thickness), is identified to have controlling influence on the roughness of interior surfaces. This combination is validated employing two materials namely Al1060 and Cu/Steel composite. In this combination, the factor d/2p followed by 1/t is found to have the greatest contribution towards the roughening of surfaces. Moreover, the value of this combination is proposed to keep low in order to produce components with good finish, say ≤ 50 mm−1 for Al10160 and 20 mm−1 for Cu/Steel composite. The present and past roughness studies in ISF are also compared, which reveals that the parameters effects are associated to the type of material. [ABSTRACT FROM AUTHOR]