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We study the atomic gas content and the central stellar mass concentrationfor a sample of almost 1500 nearby galaxies to investigate the nature ofstarbursts and the influence of galaxy-galaxy interactions on star formation(SF). We used a sample of catalogued interacting and non-interacting galaxiesin the S$^4$G survey - along with archival HI gas masses, stellar masses($M_{\ast}$), and SF rates from $IRAS$ far-infrared fluxes - and calculatedepletion times and gas fractions. We traced the central stellar massconcentration from the inner slope of the stellar component of the rotationcurves. Starbursts are defined as galaxies with a factor $>4$ enhanced SF raterelative to a control sample of non-interacting galaxies which are $\pm 0.2$dex in stellar mass and $\pm 1$ in $T$-type. Starbursts are mainly early-type($T\lesssim 5$), massive spiral galaxies ($M_{\ast}\gtrsim 10^{10}M_{\odot}$)that are not necessarily interacting. For a given stellar mass bin, starburstsare characterised by lower gas depletion times, similar gas fractions, andlarger central stellar mass concentrations than non-starburst galaxies. Theglobal distributions of gas fraction and gas depletion time are notstatistically different for interacting and non-interacting galaxies. However,in the case of currently merging galaxies, the median gas depletion time is afactor of $0.4 \pm 0.2$ that of control sample galaxies, and their SF rates area factor of $1.9 \pm 0.5$ enhanced, even though the median gas fraction issimilar. Starbursts present long-lasting SF in the circumnuclear regions thatcauses an enhancement of the central stellar density at $z\approx0$ in bothinteracting and non-interacting systems. Starbursts have low gas depletiontimescales, yet similar gas fractions as normal main-sequence galaxies. Galaxymergers cause a moderate enhancement of the SF efficiency (Abridged).

Gas fractions and depletion times in galaxies with different degrees of interaction